I saw the preview for Realms of Crawling Chaos, and it looks great. However, perusing the table of contents, there's not a sanity mechanic. No sanity mechanic in a Cthulhu supplement? Is this heresy?
To answer, let's take a look at the role of sanity mechanics in the two Cthulhu games I've played - Call of Cthulhu and Trail of Cthulhu.
The Call of Cthulhu (COC) Style
COC comes at sanity from the perspective that there are truths to the real nature of the universe so awful, it shatters the mind. The more you learn the truth, the less sane are you. As your Mythos Knowledge skill increases, your maximum sanity steadily dwindles.
I find the sanity mechanic is a big fail. It's hard to role play an insane person at the table; most of the time players try it, it devolves into camp. Funny, but not scary. Sanity can act as a resource pressure; it's a precious resource (like hit points), and players want to preserve it and stay in the game. Resource pressure keeps them on their toes and can be an instrument of horror.
Unfortunately, COC sanity has the exact opposite effect of reinforcing the genre; instead of plunging forward like an intrepid Lovecraftian delver who goes too far and learns the awful truths of the universe, the sanity mechanic in COC actually discourages learning those awful truths.
The Trail of Cthulhu (COC) Style
Okay, how about TOC? TOC maintains a watered down sanity mechanic, but its primary mechanic is something called Stability. Stability more or less functions like mental health - mental hit points. Any scare, fright, or eldritch horror makes an attack against your character's mental health. As stability erodes, the character's ability to function and use skills decreases.
However, the stability mechanic is counter-balanced by a mechanic called a drive. A drive is that thing that causes the character to willingly embrace the tropes of horror - to go alone into the dark basement, to keep reading the evil book, or to urge everyone forward to see the thing on the doorstep. When a character allows their drive to push them into harm's way, they get a mechanical benefit - they heal some stability back because they're following their inner muse.
The combination of stability and drives emulates how characters in a Lovecraft story behave better than the straight sanity mechanic. In a COC game, everyone knows something creepy is in the basement, and because of the potential sanity loss, they react as gamers - "I'm not going in there - no way. Let's come back in the morning." In a TOC game, everyone knows something creepy is in the basement, it's midnight and the moon is out, but everyone wants to gain some stability back by following their drives - "I feel compelled to press onward and see what's making that noise..."
The D&D Approach
Here's the real question though - do any of these approaches belong in Dungeons & Dragons? D&D is primarily about exploration and recovering treasure; at least, that’s how characters are rewarded by the system. Monsters are only an obstacle; fighting is often the least preferred option.
Would D&D benefit from a sanity mechanic that discourages exploration? (No)
Does D&D need an additional mechanic (like drives) that encourages exploration? (No)
Dungeons & Dragons characters are already adventurous and inured to awfulness. You don't need a sanity mechanic to tell you when to act scared. There are plenty of other ways to add horror to your D&D game.